In order to record or reproduce information on an optical information recording medium, it is necessary to control the position of an object lens so that the focal point of the object lens is on an information recording surface and does not deviate from a track. For such a position controlling, there is provided an object lens driving device of a so-called moving-magnet type in which a magnet is mounted on a movable body that supports the object lens. An example is disclosed in Japanese Provisional Patent Publication No. 7-19388 (pages 2–3, FIGS. 1 and 2).
In a conventional object lens driving device of the moving-magnet type, the movable body is supported by a support shaft planted on a base so that the movable body is movable along the support shaft and rotatable about the support shaft. A focusing coil and a tracking coil are attached to the base so that the focusing coil and the tracking coil face the magnet. Further, a spring member for determining a neutral position of the movable body is provided on the base. In order to perform the correction control of the focusing error, the movable body moves along the support shaft by means of an electromagnetic force generated by current flowing in the focusing coil. In order to perform the correction control of the tracking error, the movable body rotates about the support shaft by means of an electromagnetic force generated by current flowing in the tracking coil. When the currents supplied to the focusing coil and the tracking coil are stopped, the movable body returns to the neutral position by the action of the spring member.
However, in such a conventional object lens driving device, it is necessary to provide the spring member for determining the neutral position of the movable body, and therefore the number of parts increases. As a result, the cost increases and the assembling becomes complicated.
Moreover, since the magnetic circuit is constituted by a magnet only, the magnetic flux density acting on the respective coils is small, and it is difficult to generate a large electromagnetic force. As a result, in order to obtain a sufficient responsibility (acceleration) of the movable body, there is a problem that a electric power consumption becomes large.